1. Field of the Invention
The invention relates to a device for reducing the gap between a static part and a movable part in an apparatus operated at cryo temperatures.
2. Discussion of the Related Art
Because gas density varies inversely with temperature, it is desirable to dispose of mechanical vacuum pumps when operating at very low temperatures, referred to hereafter as cryo temperatures. For example, the density of a given gas increases by a factor 15 when cooled down from 300 K. to 20 K.
Mechanical pumps for high vacuum appliances require very small gaps between the movable and the static parts in order to reduce the back-flow of gases in the pump. If the pump is to be operated at low temperatures, these gaps must be extremely small (for example between 1 mm and 0.1 mm), since the back-flow increases with decreasing temperatures due to the low gas viscosity and due to the fact that the short free path at those low temperatures results in a laminar flow through the gap between the static and the movable part. Moreover, the mass flow is increased due to the higher density.
On the other hand, it is difficult to maintain small gaps between static and movable parts, especially at cryo temperatures, because of the difference in thermal expansion of the components of the pump during the cooling down or warming up. Thus, it would be useful to find a construction which ensures acceptable gaps during the temperature cycles, and which ensures the smallest possible gaps during the cryo-operation. In any case, it must be avoided that the movable and the static parts touch each other and become damaged.
From document U.S. Pat. No. 3,836,156, a method is known for reducing the gap between a static and a movable part. In this case, one of the parts is covered by a metallic wear coating which melts at a temperature slightly above the operation temperature, that means that it melts even if the temperature increases only slightly due to the rubbing contact between the two parts. However, this method is not susceptible to reduce variable gaps (for example due to thermal expansions).
Moreover, it has been proposed in EP-A-0 451 669 to coat the gap surfaces by a pasty material which is abraded during the running-in phase by the movable part.